Linux on Windows machine

Time to time I'm thinking about running some Linux distribution running on my PC.
Honestly, I'm too lazy to use just Linux as my only Operation System. I think after some time on Windows we all become "Win-depended".
So the best way for me is dual operation system. But some kind of dual-boot is not flexible enought.
I've picked VMware Player and Ubuntu Linux distribution. The whole OS should be running in the single windouw under MS Windows.
I haven't tested it yet but I'm wonder how much memory I need to run it.